Copper base alloys and the method of treating the same to improve their machinability



1964 H. L. BURGHOFF ETAL 3,158,470

COPPER BASE ALLOYS AND THE METHOD OF. TREATING THE SAME TO IMPROVE THEIR MACHINABILITY 3 Sheets-Sheet 1 Filed May 8, 1961 1 Micrwtmcture of typical Fig. 2 Microstructure of typical commercial free-cutting brass commercial free-cutting brass rod showing Type 1 inclusion. Polished,

unetched. (X300) (Inclusion positioned between diamond-shaped locating marks) HAROLD w H H RICHARD M. WINTER HENRY L. BURGHOFF E Fig. 3 Microstructure of typical g gg fgg g R commercial free-cutting brass rod showing Type 1 inclusion. Polished, unetched. (X300) (Inclusion INVENTORS positioned between diamond-shaped locating marks) 24, 1964 H. BURGHOFF ETAL 3,153,470

COPPER BASE ALLOYS AND THE METHOD OF TREATING THE SAME TO IMPROVE THEIR MACK-[INABILITY Filed May 8, 1961 3 Sheets-Sheet 2 Fig. l Micro Specimen of Type 1 inclusion. Polished, unetched. g- 5 Micro Specimen of Type 1 inclusions. Polished, unetched.

Fig. 6 Micro Specimen of Type 1 (lower right) and Type 2 (upper left) inclusions. Polished, unetched. (x1000) Fig. 7 Micro Specimen of Type 2 inclusions. Polished, unetched.

HAROLD W. HUGHSON RICHARD M. WINTER HENRI L. BURGHOFT F. GERALD PARKER Fig. 8 Micro Specimen of Type 2 DAVID MENDS inclusion. Polished, unetched. INVENTORS (x1000) ATTORNEYS Nov. 24, 1964 Tool Life Hours Filed May 8,

. H. L. BURGHOFF ETAL 3,158,470 CQPPER BASE ALLOYS AND THE METHUD 0F TREATING THE SAME TO IMPROVE THEIR MACHINABILITY 1961 3 Sheets-Sheet- 5 Tool Life- Silica Inclusions lmmz U) 3 O I Standard Free-Cutting Brass Rod (Silica Free) 6) i E 3 F I G 9 O I l- 0 o L I l 0 2 4 6 8 IO l2 Silica Inclusions /mm Tool Life Zinc Sulfide Inclusions lmm o Hamid (O-Hu lawn. 'Pi hard magi? Hen L.

- RGSEQHT'LEW TDawd M0145 l l I I I l 0 I0 20 so 40 so so 10 a0 'NVENTORS Zinc Sulfide Inclusions/W11 United States Patent Ofliceii 3,l58,47 Patented Nov. 24, 1964 COPPER BASE ALLQYS AN!) THE METi-EGD @PE' TREATENG THE SAME T9 EMERUVE THEE; TWKACHHNABELETY Henry L. Burghofif and Frederick Gerald Parker, Waterbury, (loans, assignors to Chase Brass 8. (Iopper Co. incorporated, Waterbury, Conn, a corporation of Connecticut Filed May 8, 1961, Ser. No. 119,009 8 Claims. (Cl. 755-435) This invention is concerned with improvement in the machinability of copper-base alloys, and more particularly in leaded brasses, of the type used in large quantity for producing machined parts. The invention is especially directed to copper alloys for producing rod from which such parts, commonly characterized as screw machine parts, are fabricated by repetitive operations on an automatic machine. In such operations the resistance to wear of cutting tools which results from high rates of sustained, repetitive machining operations, is a vitally important factor in achieving maximum machine production while maintaining dimensional tolerance and surface finish requirements in the parts produced. We have found that the machinability of these copper-base alloys is remarkably affected by certain hitherto unsuspected characteris tics of these alloys. As a result of this, we have found means whereby deleterious variations in machinability may be minimized and new heights of machinability ob tained.

Leaded brasses of numerous compositions have long been used in industry because of the ease with which they can be machined. The lead which is present in such alloys imparts the characteristic of free machining. In wrought alloys, lead may be present to as much as about 4%. The higher the lead content within this range, the higher or better is the machinabili-ty. Alloys of interme diate lead content represent a compromise between easy machinability on the one hand, and other types of work ability on the other hand, such as fianging, knurling, thread rolling, forming and drawing.

An alloy known generally as free-cutting brass, having a nominal composition of 60-63% copper, 2.5%- 3.7% lead, the remainder zinc except for the usual incidental impurities, is the alloy most frequently used in making parts on automatic screw machines. This composition range represents a combination of high machinability and desirable mechanical properties. Despite its long rec rd of being one of the most machinable of alloys, there have been complaints with regard to the variability of its performance, inconsistent and premature wearing of cut ting tools, consequential damage to tools and interruption of operations in order to re-grind the cutting edge or to replace the tools. Sometimes it is necessary to reduce the speed of machining in order to extend tool life or to be able to make a satisfactory part.

It has been suggested from time to time that hard inclusions, such as particles of undissolved steel or iron in the brass might be responsible for the poor performance on such occasions, and diligent searches have been made and samples of such brass carefully investigated for such inclusions. It is a known fact that foreign material such as iron or steel is often included in the metal scrap which conventionally is included in the starting charge in the melting and casting of the alloy. On rare occasions such metallic inclusions have indeed been found in the finished product. Much more often, however, no rational explana* tion of the untoward behavior has been found and gen erally the incidents have been passed olf as operational inefiiciency, poor machine set-up or personal idiosyncrasies of the machine operator.

In order to arrive at a better understanding of the ma chining characteristics of brass, we have undertaken a program to evaluate factors important to machining, includ ing: composition with respect to actual content of copper, lead and zinc, the basic ingredients; impurities which may be brought in from various sources and including elements such as iron, tin, aluminum, silicon, chromium; also the microstructure of the alloy; temper of the rod; surface finish and cleanliness of the rod; angles and feeds of tools; and speed of rotation of the rod. With the foregoing factors in mind we have used an automatic form and cutoff machine capable for unusually high maximum rota tional speeds for the determination of the machinability behavior of free-cutting brass rod heretofore generally available from various commercial sources. A variety of cutting operations including forming, drilling, reaming, balance turning and cut-off have been examined and a statistical analysis has been made of variation in diameter of successive work pieces produced by this machine from these materials and its relation to the machine capability. Consistent and rational results have been obtained in this investigation, and they are found to substantiate general experience in commercial screw machine shops. Thus, for commercial free-cutting brass generally available heretofore, a 4-hour life has been established as the normal best expectancy for a high speed steel form tool, before re-sharpening, when operating at a spindle speed of 12,000 r.p.m. and a tool feed of 0.002 inch per revolution on /2" diameter rod to produce a given work piece. It has been found, however, that the machining characteristics of different lots of free-cutting brass rod may vary widely, and in fact a lot of rod used in one test can give vastly different machinability results from another lot of rod, nominally of the same composition and production his tory, subjected to the same control test. For example, instead of obtaining the normally best expected 4-hour tool life, a life of only 2 hours, or even less, is all that is obtained with certain lots of rod and, furthermore, the tools in these instances are worn as though they have been ground with an abrasive.

Examination of the microstructure of rod giving such inferior results, in order to determine if any deleterious inclusions were present, was fruitless at first. This is not surprising in view of the difiiculty of finding minute foreign particles amidst the myriad of discrete particles of lead which are present characteristically in the alloy. Reference to FIG. 1 of the drawings will bear this out, wherein the microstructure of typical free-cutting brass, free of any beta phase, is shown in the photomicrograph. However, after persistently examining carefully prepared samples at a high magnification, on the order of 1,000 diameters, under the microscope, we have discovered that there are, indeed, inclusions present in conventional free cut-ting brass that can not be rationalized from knowledge of the ordinary chemical analyses of composition of the alloy. These inclusions generally range in their longest dimension from something less than 1 micron to as much as 50 microns. Application of a Bergsman microhardncss tester to these fine particles indicates that they are of two general levels of hardness. The first, originally classified as Type 1 and now believed to be essentially silica, SiO has a hardness about ten times as great as that of the brass itself. The other type, Type 2 and now believed to be essentially zinc sulfide, ZnS, has about two to three times the hardness of brass itself. FIGS. 2-8 of the drawings illustrate various forms of these inclusions at magnifications of 300x and 1000 Positive compositional identification of these two types of particles is extremely difi'icult but we believe that the characterization of them as silica and zinc sulfide is now reasonably accurate and assured. For convenience, there- 3 23: fore, in the discussion which follows they will be referred to by these terms.

Upon arriving at the foregoing discovery, in carrying out machining tests on numerous lots of rod obtained from a wide variety of commercial sources, we have found that tool life is definitely related to both types of inclusions. Where there is substantially no silica present and the observed average count of zinc sulfide inclusions is on the order of 20 to 40 per square millimeter, the tool life found is the best expected for normal conventional rod. Our studies clearly show that where any silica particles are present, form tool life is adversely affected. When on the average as few as two of these inclusions per square millimeter of metal surface as examined under a microscope are observed, the form tool life is drastically reduced from the best life expected in the cutting of normal conventional rod. Where the freecutting brass contains an observed average of about 70 inclusions of zinc sulfide per square millimeter of examined surface, but with substantially no silica present, the tool life is also reduced appreciably below the best expected life. But where there are substantially no inclusions, either of silica or of zinc sulfide, a remarkable increase in tool life is found amounting to from 2 to 5 times the normal best expected life for conventional freecutting brass.

For the purposes of this discussion and in the appended claims, the expressions substantially complete elimination of inclusions, or substantially complete absence of inclusions" are employed to mean the following: In the case of silica, these expressions denote that not more than one such inclusion, and preferably none at all, is observed under the conditions hereinafter explained per square millimeter of representative alloy surface; and in the case of 'zinc sulfide, not more than ten and preferably less than five inclusions are observed per square millimeter of alloy surface.

The presence of sulfur in copper alloys has in the past, never been regarded as harmful to machinability. in deed, it is customary to add as much as 0.25% sulfur to copper to form a commercial alloy whose distinguishing characteristic is improved machinability with respect to pure copper itself. In copper, this sulfur exists as discrete particles of copper sulfide. Similarly, sulfur is added to centain steels for the purpose of improving machinability. Our finding that sulfur in brass is detrimental to its machinability is therefore most unexpected and contrary to reasonable anticipation and is of very significant commercial importance.

Inclusions of one or both types described above have been found in some free-cutting brass rod of all of the various sources of supply tested, indicating their widespread and uncontrolled occurrence in commercial material. Visual identification of the inclusions can be made provided a magnification of at least l000 is employed. Using this order of magnification, the silica and zinc sulfide inclusions can be visually identified by their shape, color, location and size. In the case of the silica inclusions, the shape is usually oblong or irregular, ranging, as indicated above, in maximum dimension from about l micron to 50 microns. The silica inclusions are dark blue-gray in color and stand out in relief when polished. Commonly there are imperfections in the inclusions and their surface is pitted. The silica inclusions are never spherical which helps to distinguish them from the zinc sulfide inclusions.

The latter are generally spherical or geometrical in shape and sometimes have a tail or irregularity attached to the nodule. These sulfides inclusions polish fiat and range in color from light to dark gray, often being mottled in appearance. Their distribution is apparently quite random throughout the cross-section of the rod and they range from about 1 to 5 microns in size.

Microhardness tests on the inclusions themselves vary from around 1,000 to 1,600 Knoop hardness numbers d (Kl-IN), for the silica, While the zinc sulfide inclusions range between 180 to 380 KHN.

Wherever examination of the alloy reveals an inclusion count which, on the average, indicates the presence of l silica inclusion per square millimeter of surface examined, the life of the cutting edge of the tool is reduced by about one-four as compared with the life of that tool when cutting silica-free rod. Thus it is apparent that silica inclusions in the rod have an extremely important euect on tool life. The effect of silica inclusions on tool life when forming or machining free-cutting brass is illustrated graphically in FIG. 9 of the accompanying drawings. The data plotted on this chart was obtained from machining various samples of /2" diameter freecntting brass rod using a molybdenum high speed steel form tool having a back rake angle of +2. A chemical lubricant, diluted 1:25 with water, was applied during the machining operation and the cutting speed was 1570 surface feet per minute at a feed rate of 0.002 inch per revolution.

The effect of zinc sulfide inclusions on tool life, in the absence of silica, is shown on the graph of FIG. 10 from which it is noted that tool life increases as the zinc sulfide count decreases. Zinc sulfide inclusions, however, are not as detrimental to tool life as are the silica inclusions, unless the sulfide count is greater than about 70 to inclusions per square millimeter. Quite commonly the zinc sulfide count in commercial free-cutting brass is between 20 to 40 per square millimeter.

Some appreciation of the difficulty of identifying the deleterious zinc sulfide inclusions in the brass matrix already peppered with lead inclusions can be gained when it is considered that many of these approach 1 micron in size. In spite of the smallness of these sulfide inclusions, however, as little as 0.001% sulfur in the brass can produce on the order 5 l0 particles of 1 micron size per cubic inch of metal, which explains in some measure their effect on tool life.

In order to determine the inclusion count, this is made with a microscope using a magnification of l000 over a sufiicient number of different fields to provide a representative count for a total examined area equivalent to one square millimeter. Fields near the outside surface of the material are included in order to make sure that silica inclusions, if present, will be found.

Having thus determined the cause or reason for the difficulties and variable results in machinability of the copper base alloys, the next step is to provide a cure.

We have found that the foregoing zinc sulfide inclusions and, surprisingly, the silica inclusions also can be eliminated from the alloy by the addition of a suitable reagent metal to the melt shortly before casting. Magnesium is particularly effective but other reagent metals can also remove the undersirable inclusions from brass either partially or wholly. These include calcium (preferably in the form of a copper-calcium-silicon master alloy). Manganese appears definitely not to be effective.

in the case of magnesium, useful improvements in clearing the microstructure and in obtaining better machinability are obtained with from about 0.01% to as high as 0.23% residual magnesium in the free-cutting brass. Residual magnesium of 0.35% produces hot shortness in extrusion and cold shortness in cold drawing of rod of the alloy, both operations being conventional in commercial production. The upper limit of retained magnesium practical in the brass thus appears to be in the neighborhood of 0.3%. Optimum content, both from machinability as well as economic consideration, is on the order of 0.02% to 0.18% magnesium.

The effect of the magnesium appears to be synergistic, for tool life with some magnesium-containing material is far greater than with non-magnesium treated material having low or no undesirable inclusions.

The beneficial effect of magnesium is set forth in the accompanying Table I which shows form tool life for a standard test as great as 20 hours for one magnesiumbearing rod, and pronounced improvement over commercial inclusion-containing material for all magnesium contents in the useful range.

TABLE I Efiect of Magnesium in Brass n Tool Life W DIAMETER FREE-CUTTING BRASS ROD [Nominal composition: 61% Cu, 3.2% Pb, bal. Zn]

Inclusions/111111. Magnesium, Tool Life, Rod No. percent Hours Silica Zine Sulfide 1 1. 2 9 O 2 2. 0 8 66 1. 9 5 19 2. 0 4 38 3. 2 0 80 4. 0 0 56 3. l 0 46 3. 0 0 45 3. 8 O 41 3.0 0 40 4. 4 0 38 4. 6 0 31 3. 1 0 28 4. 0 0 25 5. 7 0 0 20. 8 0 0 13. 3 0 0 0. 045 17. l 0 0 0. 059 1 17. 0 0 0 0. 080 9.3 0 0 0. 11 1O. 1 0 0 0. 16 12. 2 0 0 23 0. 17 10.4 0 0 24 0. 18 7.8 0 0 25 0.23 5. 5 0 0 Ms HEXAGONAL FREE-GUTTING BRASS ROD [Same nominal composition as above] %4" DIAMETER LEADED FLANGING BRASS ROD [Nominal composition: 62.5% Cu, 2% Pb, bal. Zn]

1 Test discontinued; tool still in good condition.

The improvement from the use of magnesium is obtained despite the fact that it, too, produces discrete particles of a phase which is also harder by 2 to 3 times than the brass in which it occurs. Such a phase will be found with magnesium contents of about 0.06% and higher.

Production of magnesium-containing copper alloy may be accomplished in several diiferent Ways. In the first of a series of heats of commercial size, the preparation of a nominal 0.2% magnesium-bearing copper alloy was accomplished from a charge consisting entirely of freecutting brass scrap. This was charged into a furnace and immediately after melt-down, with the melt relatively cool, the dross was raked off the melt and magnesium was added in the form of an 80% (In-20% Mg master alloy. The master alloy was introduced by thrusting it beneath the surface with a ladle, with the furnace power on. When the furnace reached pouring temperature, power was turned olf and the melt allowed to stand for five minutes. The melt was then skimmed and poured into a mold.

Mechanical properties, assay results, inclusion counts and machinability data for a typical Melt A prepared in this way are shown in the accompanying Tables II, III and IV.

A second series of ingots was cast from charges consisting mainly of scrap, including small amounts of chips.

Mg (percent) 0. Inclusions/mm):

Sili

T001 Life (hours)? Magnesium was again added as the copper-magnesium master alloy, and the metal allowed to stand for 2 minutes with the furnace off before pouring. The results of this are typified by the data given for Melt B in the accompanying Tables II, III and 1V.

ingots containing no magnesium addition were also poured for comparison purposes, and the properties of a representative ingot from this group are shown in the accompanying tables as Melt C.

A further series of ingots was produced with contents up to 0.35% magnesium. Hot extruded and coiled rod with 0.35% Mg was found to contain intercrystalline cracks. It also proved impossible to point the ends of the rods without further cracking them. Examination of the microstructure of this rod showed lead and both alpha and beta phases to be present, together with particles produced by the magnesium, mainly in the form of an intercrystalline network. This was probably the cause of the cracking and thus indicates definitely an upper limit for the magnesium content.

The addition of magnesium to the melt may be accomplished .by the use of unalloyed magnesium metal as Well as in the form of a master copper-magnesium alloy specifically mentioned in the foregoing examples.

The presence of magnesium in the melt exerts a marked effect on the tendency of zinc to burn during pouring. Thus it is observed that the stream of metal becomes covered with a thin skin of adherent oxide which protects the zinc from burning. The magnesium additions, although they may cause the formation of more dross than usual, tend to decrease zinc stack losses which is definitely a favorable factor.

Analysis TABLE IV Inclusion Melt Sample L Sample II From the results shown in Tables II, III and IV, it is seen that there is no significant difierence in mechanical properties, grain size or beta content as a result of magnesium additions. On the other hand, there is a marked increase in tool life in the magnesium-bearing brasses.

As previously mentioned we have found that reagent metal additions other than magnesium are useful for removing the undesirable inclusions. Specifically, calcium is useful for this purpose. In this case the calcium is most economically added in the form of a master alloy made up by melting commercial calcium-silicon (35% Ca, 65% Si) in copper, yielding an alloy containing 21% Si and 11% Ca. This alloy dissolves readily in the melt. Results obtained for free-cutting brass having retained calcium of from 0.001% to 0.05% are listed in Table V.

The brasses have long been recognized as among the cleanest alloys structure-wise of all those commercially used, by virtue of their zinc content and the purging action which this element itself exerts in the melting L operation. The beneficial effect of additions of a reagent metal under this circumstance is therefore a most unusual thing.

The invention is not limited to free-cutting brass but is applicable to commercial brasses, generally. In Table I an example of fianging brass is given, in which the nominal lead content is on the order of 2%. And the same is applicable to low-leaded brass having a nominal composition of 66.5% copper-0.5% lead, balance zinc. The latter alloy shows inclusion counts typically of 4 silica, 30 zinc sulfide inclusions per square millimeter. Nor is the invention limited to leaded brasses as it may be applied to nominally lead-free alloys, such as cartridge brass (nominally 70% copper-30% zinc), and jewelry bronze (nominally 87.5% copper12.5% zinc). The latter alloy particularly is much used in the manufacture of slide fasteners and has been found to produce large variation in the wear of the cut-off tool used in making the fastener elements. Typical inclusion counts for this jewelry bronze show up to 5 silica and to 12 zinc sulfide particles per square millimeter. The cartridge brass mentioned above shows, typically, silica counts of 6 to 7 and zinc sulfide counts as high as 75 on occasion. Removal of these inclusions from any of these alloys ,by the technique disclosed herein is beneficial.

TABLE V Efiect of Calcium in Brass on Tool Life W DIAMETER FREE-CUTTING BRASS ROD What is claimed is:

1. A leaded-brass of superior machinability which has been treated to reduce silica and zinc sulfide inclusions commonly present in the untreated metal, said treated brass containinga reagent metal selected from the group consisting of at least one of the following components in the retained amount specified: magnesium from about 0.01% to about 0.30%; and calcium from about 0.001% to about 0.05%.

2. A leaded-brass as defined in claim 1, wherein said brass composition has a nominal analysis, apart from said reagent metal, of 60% to copper, 0.5% to 3.7% lead, the remainder zinc except for incidental impurities.

3. A leaded-brass as defined in claim 1, wherein said brass composition has a nominal analysis, apart from said reagent metal, of 60% to 63% copper, 2.5% to 3.7% lead, the remainder zinc except for incidental impurities.

4. A free-cutting brass as defined in claim 1, wherein the reagent metal is magnesium in amount of from about 0.02% to 0.18% by weight.

5. The method as defined in claim 7, wherein the reagent metal added is magnesium in amount sufiicient to produce in the cast alloy a retained magnesium content of from 0.02% to 0.18%.

6. The method as defined in claim 7, wherein the amount of reagent metal added is suificient to reduce silica and zinc sulfide inclusions in the untreated alloy matrix to an average of not more than one silica and ten zinc sulfide inclusions per square millimeter of metal cross-sectional surface.

7. The method of improving the machinability of leaded brass alloys which comprises reducing the silica and zinc sulfide inclusions normally present in such alloys by preparing a melt of such alloy for casting and then introducing into said melt shortly before casting at least one reagent metal selected from the following components to provide in the as-cast alloy the respectively designated retained amount of such reagent metal: magnesium from about 0.01% to about 0.30%; and calcium from about 0.001% to about 0.05%.

8. The method defined in claim 7, wherein said copper base alloy is a brass having a nominal composition of 60-63% copper, 2.53.7% lead, the balance zinc.

References tCited in the file of this patent UNITED STATES PATENTS 1,937,934 Zimmerli Dec. 5, 1933 2,173,254 Hensel et al Sept. 19, 1939 2,879,159 Bolkcom et a1. Mar. 24, 1959 2,970,248 Sahagun Jan. 31, 1961 OTHER REFERENCES Samons: Engineering Metals and Their Alloys, The Macmillan Co., New York, 1952, p. 558. 

1. A LEADED-BRASS OF SUPERIOR MACHINABILITY WHICH HAS BEEN TREATED TO REDUCE SILICA AND ZINC SUFLIDE INCLUSIONS COMMONLY PRESENT IN THE UNTREATED METAL, SAID TREATED BRASS CONTAINING A REAGENT METAL SELECTED FROM THE GROUP CONSISTING OF AT LEAST ONE OF THE FOLLOWING COMPONENTS 